In the prior art and herein, zero Joule heating loss in wires is achieved by the use of substances known as superconductors in which the coherent motion of paired-charges proceeds in the absence of scattering by, and interaction with, the host lattice, grain boundaries, defects, imperfections and other charge carriers. A great need for such wires exists in practical applications such as long distance transmission of power, motors and generators, high field electromagnets, and superconducting magnet energy storage systems.
Until recently, it was believed that superconductivity above 23.degree.K was not possible. This belief stemmed from the theoretical work now named the BCS theory (Bardeen, Cooper and Schrieffer) which predicted such an upper limit. The temperature at which superconductivity begins in a superconductor (in the absence of any external magnetic fields) is termed the critical temperature of that superconductor and this term will be used herein.
In the early 1970's a number of theoretical proposals were presented, suggesting that the critical temperature for superconductivity could be increased. (V. L. Ginzburg, Usp. Fiz. Nauk. 101, 185 (1970)) (D. Allender, J. Bray, J. Bardeen, Phys. Rev. B8, 4433 (1973)), but the lack of any discoveries of superconductivity above 23.degree.K solidified the belief that indeed this critical temperature could not be exceeded. A significant experimental breakthrough in high temperature superconductivity (critical temperatures in excess of 23.degree.K) was provided in November 1986 by Bednorz and Muller when they published a tentative disclosure of high temperature superconductivity (Geor Bednorz and Alex Muller, Z. Phys. B64, 189 (1986)). Following this disclosure, another report cited a critical temperature above 30.degree.K for La(2-x)Ba(x)CuO(4-y), (H. Takagi, S. Uchida, K. Kitazawa, S. Tanaka, Jpn. J. Appl. Phys. 26, L123 (1987)).
Confirmation of a critical temperature of 93.degree.K was reported by Chu for yttrium-barium-copper oxide ceramic (M. K. MU, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, 2 March, 1987, p. 908.) This material was dubbed the 123 compound and has served as a model for advanced research in the field.
Since that time, there have been a large number of papers published, and a large number of well attended meetings on this subject. One source of information about high-T.sub.c superconductivity is a 400 page volume of 112 papers that were published in Physical Review Letters, and Physical Review during the first 6 months of 1987. During 1987 and 1988, a number of families of high temperature superconducting materials have been discovered with confirmed critical temperatures as high as 162.degree.K. These materials are usually ceramics containing copper (whose apparent valence state is trivalent), an alkaline metal (Ca, Sr, or Ba) and a rare earth including Yttrium. In some later developments, the rare earth has been partially or completely replaced with Bi to yield critical temperatures between 90.degree.K to 160.degree.K, or with thallium to yield similar critical temperatures.
Most of these superconductors showed some degree of anisotropy in their properties and it was therefore significant when a cubic ceramic with a critical temperature above 23.degree.K (specifically 30.degree.K) was discovered based on a complex oxide of Ba, K and Bi. This superconductor was significant in that it was the first high temperature superconductor without copper in its composition, thus indicating that the occurrence of high temperature superconductivity may be more prevalent than originally realized. Amorphous high temperature superconductors have also been reported based on the bismuth compounds in which some of the bismuth was replaced with lead. The critical temperatures and critical current of these amorphous superconductors are somewhat lower than those of their crystalline counterparts.
There are some scattered reports of superconductivity above 162.degree.K. For instance, R. G. Kulkarui has reported superconducting oxides having an approximate composition CaO(0.5)ZnO(0.5)Fe(2)0(4), with critical temperatures in this range. Ogushi also reported superconductivity at room temperature in yet ill-defined niobium strontium lanthanum oxides. While these reports have yet to be confirmed independently by other researchers, it is reasonable to expect that superconductors operating at room temperature will soon become available.
In the prior art concerning superconducting wires, which is mostly based on the A15 class of intermetallic compounds of niobium with tin, germanium, or vanadium, ultra fine filaments of the superconducting materials are encased in a copper matrix. The process of manufacturing these multifilament wires is difficult and expensive since most intermetallic compounds are quite brittle, and thus are not easily reduced to small diameter wires by any of the prior art extrusion processes.
Bulk superconducting ceramic compositions are normally prepared by weighing out specific quantities of selected oxides. The combination is thoroughly mixed by conventional means and then fired at temperatures sufficiently high to induce solid state reaction and sintering of the desired ceramic composition and structure. In many of the copper based compositions, further annealing in pure oxygen is shown to improve the superconducting properties by enhancing the trivalent state of copper, believed to be essential to the high temperature superconducting phenomenon.
The above method for producing superconducting compositions is limited in the physical form of the superconductor so-produced and cannot be easily used to manufacture mechanically and electrically acceptable wires. Other methods such as the electron-beam evaporation method or the vapor phase epitaxial growth method can only produced thin films. Even the method which employs laser-melting of a ceramic bar to form single crystal whiskers, while yielding material with high current density capabilities, cannot be used to manufacture a continuous wire. The slurry process involving the oxides precursors followed by appropriate calcination and oxidation could possible be used for wire manufacturing, but the critical current densities achieved are still too low to be acceptable.
The new superconducting ceramics are brittle, and efforts to increase their strength and flexibility in bulk form have not been successful heretofore. Many workers in the prior art have tried to produce a "wire" directly from the superconducting ceramic itself, or by the oxidation of a precursor alloy. These result in wires that are mechanically weak with low current carrying capability. Once these superconductors have become known, it has been obvious to those skilled in the arts that if one desired to optimize the current carrying properties of the new ceramic superconductors, certain parameters needed to be controlled. For example, an anisotropy of critical current has been observed, and the indications are that maximum current density is conducted in a plane perpendicular to the "C" axis of the crystal structure in certain of the compounds. Another factor is that the coherence length of the charge carriers in these high temperature superconductors appears to be very small, of the order of 20 to 50 angstroms (a few lattice parameters).
Most workers agree that minute impurities do not have a major impact on transition temperature and critical current of these superconductors, providing that said impurities are well dissolved within the matrix. On the other hand, agglomeration at grain boundaries and the formation of important nonsuperconducting barriers at grain boundaries probably has a serious and deleterious effect on overall current carrying capabilities. This is particularly important if the extent of the "impure" grain boundaries exceeds the coherent length.
I have determined that the prior art multi-filament approach to produce superconducting wires from the new ceramic superconductors has a number of intrinsic shortcomings. I have therefore developed a completely new approach to the manufacturing of long and continuous superconducting wires of the new ceramic high temperature superconductors.
For instance, a large mass of copper is usually required in which to embed transitional superconducting filaments so as to support the current carrying capacity required in the event of accidental quench of the superconductors. This approach is required since traditional superconductors operate at only few degrees under their critical temperature. Thus, very small perturbations and temperature excursions can cause the small temperature rise necessary to quench the superconducting state. Furthermore, the large mass of copper also has the function of providing mechanical support for the superconducting filaments.
With the advent of high temperature superconductors, I have found that the combination of higher heat capacity (100 times larger than traditional superconductors) and higher allowed temperature excursions (up to 10 times larger), negates the need to provide for a copper matrix for quench protection. I have also found that, in some applications, this large mass of copper is actually detrimental when alternating currents and other electrical perturbations prevail.
I have found that a much better solution to producing a wire having superconducting properties is to form a composite wire consisting of up to four concentric layers, namely a supporting core, an interlayer (metallization), an active (superconducting) layer and an outer metallization. This form is much more flexible than any known in the prior art and is easier to produce as well. Therefore, it is an object of this invention to provide methods of forming superconducting wires having markedly improved physical and electrical properties over those of the prior art.
Another object is to provide methods of forming said wires where bonded layers are formed, each of which has a specific function. Still another object is to provide a wire having a supporting core combined with a superconducting layer and an outer metallized layer. One other object is to provide a wire whose superconducting layer is composed of columnar grains with their "c"-axis perpendicular to the wire's axis. Another object is to provide methods whereby the grain structure of the so-deposited superconducting layer is composed of grains with a relatively homogeneous size distribution. Still another object is to provide methods whereby the so-deposited superconducting layer is amorphous. A final object is to provide a wire with superconducting properties that is much more flexible, and that will carry supercurrents at much higher densities than feasible by other techniques.